1. Field of the Invention
The present invention relates to a radio telecommunication system such as a portable radio telephone system or a vehicle telephone system and, more particularly, to a system employing a TDMA scheme as a radio channel access scheme.
2. Description of the Related Art
As a system of this type, for example, a cellular radio telephone system is available. FIG. 10 shows a schematic arrangement of a cellular radio telephone system.
This system comprises a control station CS, a plurality of base stations BS1 to BSn, and a plurality of mobile stations PS1 to PSm. The control station CS is connected to a wire network NW. The plurality of base stations BS1 to BSn are connected to the control station CS through wire lines CL1 to CLn, respectively. The base stations BS1 to BSn form radio zones E1 to En in different areas. The plurality of mobile stations PS1 to PSm are connected through radio channels to the base stations in the radio zones in which they are located and are further connected to the wire network NW through the base stations and the control station CS.
This system employs a multichannel scheme for accessing radio frequencies. In the multichannel access scheme, a plurality of speech radio frequencies and a small number of control radio frequencies are shared by the base stations BS1 to BSn and the mobile stations PS1 to PSm. When a calling signal is generated by a given mobile station or a ringing signal is received from the wire network NW, a free speech radio frequency and a free control radio frequency are selected from these speech radio frequencies and control radio frequencies, thus performing radio communication between the mobile and base stations.
Assume that a calling operation is performed by a given mobile station. In this case, in the mobile station, a free control radio frequency is selected from the small number of control radio frequencies. The selected control radio frequency is used as a control channel, and communication of a control signal associated with a calling or ringing operation is performed between the mobile station and the corresponding base station. At this time, in the control station, a free speech radio frequency is selected from the plurality of speech radio frequencies. Thereafter, radio channel designation information representing the selected free speech control radio frequency is inserted in the control signal to be transmitted from the base station to the mobile station. Subsequently, speech communication can be performed between the mobile and base stations by using the free speech radio frequency as a speech channel.
In a system of this type, the most important subject is associated with an increase in office service capacity.
Under the circumstances, a TDMA scheme has recently been proposed as a scheme for accessing radio channels between base stations and mobile stations. In the TDMA scheme, a signal to be transmitted at each speech radio frequency is formatted to have a time frame. One time frame is constituted by six time-divisionally multiplexed time slots. The time slots of each speech radio frequency are respectively assigned, as speech channels, to mobile stations. The respective mobile stations perform radio communication with corresponding base stations by using the time slots assigned to them. If this TDMA scheme is employed, the office service capacity can be increased six times that of a conventional system.
In a radio tele-communication system using such a TDMA scheme, the following problems are posed. In a system of this type, the same speech radio frequency is sometimes used in a plurality of base stations, e.g., the base stations BS1 and BS2 at once. In this case, the mobile station PS5, which is located in an area where the radio zones of the base stations BS1 and BS2 overlap, may receive a wave from the base station BS2 in addition to a wave from the base station BS1 with which the mobile station PS5 communicates.
In some system, the base stations BS1 and BS2 are asynchronous with each other. In such a case, while a given mobile station performs a speech communication with the base station BS1 for a long period of time, a relative positional shift may occur between the current used time slot and an adjacent time slot transmitted from the other base station BS2 due to the precision difference between clock frequencies used in the base stations BS1 and BS2, resulting in interference between the time slots. If such interference occurs, errors are caused in speech communication data, and the quality of speech communication greatly deteriorates. If the degree of interference is high, the mobile station cannot detect a sync signal inserted in a time slot. As a result, a step-out occurs, and the speech communication may be interrupted, thus posing a serious problem.
In order to solve this problem, the clock precision of the base and mobile stations may be improved or a sufficient guard time may be set between time slots to absorb the positional shift between the time slots. The former method leads to an increase in cost of each station circuit. In the latter method, the transmission efficiency is decreased.